1. Field of the Invention
This invention relates generally to the repair of heat-exchangers, and more particularly to the restoration of individual tubes of such exchangers. Exchangers of the type to which this invention pertains are routinely employed on ships and submarines, also by commercial utility companies in connection with the generation of electrical power, and in oil and chemical refineries, as well as water de-salinization plants.
In particular, the invention addresses the problem of restoration of complete lengths of individual tubes extending between two oppositely-disposed tube sheets of a heat-exchanger or condenser unit.
2. Description of the Related Art Including Information Disclosed Under 37 CFR .sctn..sctn.1.97-199
Heat-exchanger tubes which are subjected to wear and corrosion over extended periods of use eventually develop stress-related cracks or areas of weakness in the tube walls. Over the years, it has been discovered that a large majority of such failures occurs within the first three to six inches of the end of the tube. Such failures are thought to result from water turbulence which is present when high-velocity cooling water that is laden with chemical impurities and air/gas bubbles for cibly impinges on the wall surfaces adjacent the inlets of the tubes. The nature of the damage is both physical (abrasion and cavitation), and electro-chemical (including galvanic attack)
An example of a prior method of restoration of worn tubes is illustrated and described in applicant's U.S. Pat. No. 4,941,512 dated Jul. 17, 1990 and entitled METHOD OF REPAIRING HEAT EXCHANGER TUBE ENDS. This patent discloses the use of a tubular insert or liner which is placed inside the eroded end section of the exchanger tube to be repaired The liner is then forcibly expanded into intimate physical and heat-exchanging contact with the damaged wall of the tube, thus restoring the end section to a useable condition. Preferably the liner is then flared at its outermost end, to simulate the original tube configuration at the location of its joint with the tube sheet.
U.S. Pat Nos. 1,937,600; 2,157,107; and U.S. Pat. No. 3,317,222 disclose various arrangements involving protective tubular liner inserts which are intended to minimize the long-term destructive effects of corrosion and wear. In each case a reinforcing sleeve is fitted inside the respective end of the exchanger tube.
U.S. Pats Nos. 3,781,966; 3,962,767; 4,505,017; and U.S. Pat. No. 4,637,436 disclose methods of repairing worn end sections of heat exchanger tubes. In particular, U.S. Pat. No. '966 involves the use of explosively expanded sleeves, preferably inner and outer sleeves which are expanded into tight engagement with one another and with the inner surface of the worn exchanger tube. U.S. Pat. No. '767 discloses severing the worn tube and welding a sleeve inside such tube at the location where the severing occurred. U.S. Pat. No. '017 illustrates the use of a sleeve insert which is expanded to form a bulge that in turn causes a similar bulge in the exchanger tube wall, followed by a forcible shifting of the insert inwardly to form a seal between the outer surface of the insert and the inner surface of the exchanger tube wall. U.S. Pat. No. '436 employs a heat responsive expandable driver constituted of a shape-memory alloy, which effects expansion of a tube liner into firm engagement with the inner surface of an exchanger tube.
Finally, U.S. Pat. No. 4,694,549 illustrates and describes a procedure for replacement of an end section of an exchanger tube by cutting out a portion of the tube and replacing it with a tube liner piece that interfits with the existing tube end, expanding the new tube piece into engagement with the opening in the tube sheet and thereafter expanding the inner end of the tube piece into engagement with the walls of the existing tube end. The inner end of the tube piece is then welded to the existing tube end, and the outer end of the tube piece is welded to the tube sheet.
While some of the methods and apparati noted above have met with commercial success, the use of inserts for repairing existing tubes has certain limitations. In particular, where liners are pressed into place there arises a question of the integrity of the resulting joint. It is considered essential that the repaired tubes have the high reliability characteristics of a new tube, and be capable of prolonged operation under the relatively harsh conditions to which they are subjected when the exchanger is returned to service. In the event one or more repaired joints prove to be faulty, the exchanger often needs to be completely shut down, and the leaky tube either re-repaired or else plugged at the tube sheet. Plugging, while not considered all that objectionable when applied to a few tubes, does become a consideration when the number of plugged tubes increases. A plugged tube is completely inoperative from the standpoint of heat exchange; thus, a condition where a large number of tubes is plugged has the effect of significantly reducing the overall efficiency of the exchanger.
In addition, where welds are employed in conjunction with inserts or liners, the same question of integrity of the joint arises, namely is the weld adequate to withstand subsequent operation of the exchanger over prolonged periods, without premature breakdown or failure?
Finally, where the condition of an individual tube has deteriorated to the extent that little or none of the remaining tube is viable, restoration by means of end liners or inserts is not possible since this type of repair is applicable to only the end sections of such tubes.
Typical heat exchangers can contain hundreds of individual tubes ranging in lengths of up to 50 feet or more. In practice, replacement of entire tubes has been most difficult to achieve because usually there is not adequate space outside the exchanger in which to position a straight section of replacement tube and pass it through the original tube. This space limitation is especially severe in marine environments, such as on steamships or submarines. In general, complete replacement of individual tubes has not been possible in such cases; failures in individual tubes were handled by installation of tube liners as noted above, or alternately the entire tube was plugged (and thus taken out of service) in the event that a suitable repair by means of inserts or liners was not possible.